Semi-coincidence detector



Jan. 9, 1962 A. PETRIW SEMI-COINCIDENCE DETECTOR Filed Feb. 5, 1959 2Sheets-Sheet 1 JNVENTOR, ANDREW PETE/W.

BY W W A TTORA/EX Jan. 9, 1962 lw 3,016,490

SEMI-COINCIDENCE DETECTOR Filed Feb. 5, 1959 2 Sheets-Sheet 2 450v. F/G.2 N56 150 V, 450 V 450 V. 450 V. 450 V.

IN V EN TOR, ANDRE W PE 7/? H 3.

A TTOR/VE K United States Patent 3,016,490 SEMI-COINCIDENCE DETECTORAndrew Petriw, Spring Lake Heights, N.J., assignor to the United Statesof America as represented by the Secretary of the Army Filed Feb. 5,1959, Ser. No. 791,504

1 Claim. (Cl. 328146) (Granted under Title 35, US. Code (1952), see.266) The invention described herein may be manufactured and used by orfor the Government for governmental purposes, without the payment of anyroyalty thereon.

This invention relates to coincidence detectors and particularly tosystems for detecting coincident signals in the presence of considerablenoise in multiple channel communications. More particularly thisinvention relates to a device for detecting the presence of coincidentsignals in the majority of the channels of a multiple channelcommunication system.

The conventional approaches to the problem of detecting signalscoincident in multiple channels rely on the addition of all of thesignals in all of the channels so that an output signal is strengthenedby the coincidence of several signals in dilierent channels While randomimpulses are, comparatively, attenuated by the lack of coincidence ofcorresponding impulses in other channels. On the other hand an extremelystrong signal in one channel would produce an output which might be aslarge as the combined outputs of a plurality of smaller coincidentsignals.

A system to overcome this difiiculty when it occurs in pairs of channelsis described in the Patent No. 2,934,643, issued to the same inventor ona Coincidence Detector. In this patent a system is described whereincoincident signals on both of a pair of channels are added in the usualway while non-coincident signals produce a negative counterpart of theoriginal in a difference detecting circuit which cancels the original inthe final adding circuit. In other words non-coincident signals,regardless of their relative strength, are not only comparativelyattenuated but are actually cancelled. This system applies only to pairsof channels.

However, in certain situations more than two channels may be providedand the usable signal may be so weak that it may be necessary ordesirable to detect partially coincident or semi-coincident signals thatmay appear on a majority of the channels. A series of systems toaccomplish this are taught in the Patent No. 2,934,644, issued to thesame inventor on a Semi-coincidence Detector, wherein several of the twochannel coincidence detectors are combined and interconnected amongvarious combinations of pairs of channels in order that asemicoincidence of a co-incidence of the majority of channels, can bedetected.

These latter systems will pass signals occuring simultaneously in themajority of channels and will also eliminate a substantial part of thenoise, or other non-coincident impulses that do not occur simultaneouslyin a majority of the channels. This performs a valuable function inmultiple channel reception and provides a considerably better signal tonoise ratio than systems that merely add the coincidence of usefulsignals and rely on the randomness of noise signals to attenuatethemselves. However, these systems require considerable equipment,particularly where a large number of channels are being sampled, andwould require highly complex combinations where more than four channelswere involved.

It is therefore an object of this invention to provide an improvedsemi-coincidence detector. It is a further object of this invention toprovide an improved semicoincidence detector having a maximum number ofinputs with a minimum of circuitry. It is a further object of thisinvention to provide an improved semi-coincidence detector having amaximum number of inputs with a minimum of circuitry. It is a furtherobject of this invention to provide an improved semi-coincidencedetector for detecting the coincidence of signals in the majority of thechannels of a multiple channel system and reducing the noise impulsesthat are not coincident in a majority of the channels of the system.

This and other objects are accomplished by applying a plurality ofchannels, that are intended to carry coincident signals in the presenceof a very considerable noise, to a difference detecting circuit as wellas to an adding circuit. The output of the difference detecting circuitis applied out of phase to the same adding circuit to cancel, orsubstantially reduce, the effect of non-coincident signals. Signalscoincident in the majority of the channels, on the other hand, will beadded in the normal manner in the adding circuit without appreciableattenuation from the difference detecting circuit.

This invention will be described in more detail and other and furtherobjects of this invention will become apparent from the followingspecification and the drawings of which FIG. 1 shows a circuit diagramof a three or more channel system of one type and FIG. 2 shows a circuitdiagram of a five or more channel system of a slightly dilferent typefor accomplishing semi-coincidence detection.

Referring now more particularly to FIG. 1, three inputs 10, 11, and 12are provided to accommodate three channels that will presumably becarrying the same signals simultaneously. These inputs are connecteddirectly to the grids 51, 71 and 91 of the tubes 50, 70 and of an addingcircuit. The same inputs are directly connected to the grids 21, 31 and41 of the tubes 20, 30 and 40 that are connected to function as a formof difference detecting circuit. The outputs of these last tubes areconnected through diodes '120, and 140 to the grids 61, 81 and 101 ofthe tubes 60, 80 and 100 that are also a part of the same addingcircuit.

The adding circuit has a common load impedance 55, which is connected tothe plates 54, 64, etc., and an output terminal 58. The adding circuitis completed by the cathodes 52, 62 etc., connected through cathoderesistances 53, 63 etc., to the ground terminals 57 etc.

In this drawing similar elements have been similarly numbered forconvenience in describing the elements and their duplication offunction.

The difference detecting circuit has the cathodes 22, 32 and 42 of thetubes 20, 30 and 40 connected together and to ground 27 through a commonload resistor 23. The plates 24, 34 and 44 are connected to a source ofpotential 26, 36 and 46 thru plate load impedances 25, 35 and 45. Acommon source of potential may be used to supply these three plateimpedances but separate connections are shown here to reduce the numberof line crossings and irrelevant detail. The plates 24, 34, and 44 areconnected thru the diodes 120, 130, and 140 to the appropriate tubes ofthe adding circuit.

The diodes are loaded by the potentiometers 125, and 145, coupled to acommon source of voltage which acts as a bias voltage for the grids 61,81, and 101 as well as for the diodes. The cathodes 122, 132 and 142 andthe plates 124, 134 and 144 of the diodes are so connected that, in thiscase, where only positive input pulses are to be considered, only thenegative signals appearing in the outputs of the difference detectingtubes will be applied to the corresponding grids of the adding circuit.tubes.

It is noted that grid biasing voltages and conventional grid resistorsare necessary to maintain the grids of the several vacuum tubes in bothof the drawings at their prescribed operating levels. The requirement ofproper grid biasing and the necessary values thereof are well known inthe art and need no detailed description here. The biases of the grids21, and 51, 31 and 71 and 41 and 91 may also be provided by the directcoupling of the input channels 11 and 12, if the potentials of the inputchannels are of the proper relation with respect to other potentials ofthis circuit to make this possible.

In operation, the diflerence detecting circuit resembles theconventional forms and has a cathode impedance of as high a value aspossible to maintain a substantially constant current condition in thispart of the circuit. The cathode level is maintained at the mean levelof the input channels and all three grids can vary over a fairly widerange without a substantial change in the grid-cathode voltagerelationship and with a minimum corresponding change in the platecurrent or the voltage across the plate resistors 25, and 45. This isthe result of the constant current characteristic of this circuit. Theresistor 23 could be replaced by a triode or a pentode circuit suitablyconnected and of adequate current carrying capacity to provide evenbetter constant current characteristics.

The adding circuit is conventional and is shown with siX active inputsfor the proper accommodation of this version of applicants device. Thevacuum tubes 50-100 of the adding circuit are supplied from a source ofpotential 56.

Although three inputs are shown in FIG. 1 a fourth input 13 and certainother dotted connections indicate that additional inputs can beprovided, identical to any of the three shown, to accommodate additionalchannels. Three inputs have been shown for simplicity but any number ofinputs may be incorporated in this circuit. The only actual changesnecessary for adding more input stages to the difference detectingcircuit or the adding circuit would be in the common load impedance 55and the common cathode load 23 to accommodate the increased current. Thepower supplies would obviously have to be adequate for the current thatwould be required.

If three signals are coincident the three grids 21, 31 and 41 are drivenpositive in unison. The cathodes 22, 3'2 and 42 follow the grid actionand no substantial change in voltage across the plate loads 25, 35 and45 results. There will be no appreciable negative signal applied at thediodes 120, 130 and 140 or to the girds 61, 81 and 101 of thecorresponding adding circuit tubes. On the other hand, these positivepulses on 10, 11 and 12 are also applied directly to the grids 51, 71and 91 of the adding circuit and the combined effect of these threesignals will produce a substantial impulse across the plate load 55.

'If a signal is received on only two of the channels, the cathodes ofthe difference detecting circuit will follow the two active grids forthe most part, but some increase will result in the grid cathode voltagerelationship since the grid having no signal will hold back the cathodesto some extent. Consequently negative going pulses will be developedacross the corresponding plate loads. These negative going pulses willbe passed by the corresponding diodes and applied to the adding circuitto somewhat attenuate the semi-coincident positive signals. However, thepositive input signals applied directly to the adding circuit aresubstantially greater and will still produce a substantial output. Theinput grid that had no signal will produce a substantial positive goingpulse at the output of its difference detecting tube but this pulse willbe blocked by the diode and have no effect on the common adding circuit.

In the event that only one impulse is receivedas in the case of noise-ononly one channel, the cathode following action in the differencedetecting circuit will be a minimum, and a very substantial negativepulse will appear across the plate load of the difference detecting tubewhose grid received the impulse. This negative pulse will be passed bythe corresponding diode and applied to the adding circuit tube toeffectively cancel the original impulse or noise-signal. Thus the outputof a semi-coincidence detecting circuit as shown in FIG. 1 will have asubstantially better signal to noise ratio than that of simple additioncircuits relying solely on attenuation by averaging.

Another circuit for semi-coincidence detection is shown in FIG. 2. Thiscircuit has fewer parts, relative to the number of channelsaccommodated, than the circuit of FIG. 1 for the same random impulsecancellation. In 16. 2 the inputs 210, 211, 212, etc., correspond to 10,11, and 12 of FIG. 1. Five inputs are shown here although, as in FIG. 1,there is no limit to the number of inputs that could be employed. Thedifferences detecting circuits are in the tubes 220, 230, 240 etc.,which perform generally the same functions as those of the correspondingtubes of FIG. 1.

The main difference between FIGS. 1 and 2 is in that the outputs of thediodes 320 to 360 have been combined in a common load potentiometer 325which is tapped to connect to the grid 301 of a single tube 300 in thecombined adding circuit. This is possible since it does not matter whichdifference detecting circuit tube generates the signal-cancellingnegative pulse as long as it is applied at its correct level to theadding circuit. The positive pulses generated in the differencedetecting circuit are cancelled by the diodm as before.

The incidence of a single positive pulse at one of the inputs will stillproduce a negative pulse across the output load but the single addingcircuit tube will function for any one of the difference detecting tubesand the negative pulse is applied to the single tube in the addingcircuit instead of requiring a separate tube in the adding circuit foreach of the difference detecting tubes. Each of the inputs 210, 211,212, etc., is connected directly to the grid of a tube in the addingcircuit and to a grid of one of the difference detecting tubes asbefore.

In operation the cathodes of the difference detecting tubes will followthe potential change of the majority of the input signals applied to thegrids. If the majority of the grids receive signals the cathodes willrise and the constant current characteristic of this circuit will resultin a minimum change in the grid-cathode voltage relationship and aminimum change in the voltage drop across the corresponding loadresistors. If one of the channels does not produce a signal thecorresponding tube will draw less current, and a positive voltage changewill appear across its plate load. The positive voltage change will beblocked by the corresponding diode so that it will not affect thecoincident signals appearing in the majority of the channels of theadding circuit. On the other hand if the majority of the channels haveno input signal and an impulse appears in only one channel thecorresponding difference detecting tube will draw considerable currentand a negative pulse will appear across its plate load resistor. Thenegative pulse will pass through its diode to the common adding circuittube to cancel the single positive impulse.

In both of the circuits it is necessary that the negative, correctingvoltage produced by the difference detecting circuit be equal andopposite to the original, noncoincident, positive going signal at theirgrids of the adding circuit tubes. The negative, correcting voltage mayhave to be adjusted for best results since the circuit parameters andtube characteristics may vary appreciably. The diode loads are providedwith variable potentiometer taps in both circuits as a simple means ofcontrolling one set of the voltages. Other methods for controllingvoltages are well known and could also be applied here.

The magnitude of the input signals is limited by the amount ofgrid-cathode potential variation that the difference detecting tubes cantolerate without overloading. The adding circuit tubes can accommodate amuch greater input signal variation because of the cathode followeraction of the individual stages. Therefore, to bring this circuit to ahigher efiiciency, voltage dividers can be used to reduce the inputsignal at the grids of the difference detecting tubes as compared withthe signal at the grids of the adding circuit tubes. This reduction canbe compensated for by increasing the gain of the difierence detectingcircuit.

While the typical circuits described herein use vacuum tubes it will beobvious that the same principles and circuitry can be applied totransistors.

In the typical circuit shown in FIG. 1 the adding circuit triodes wouldbe halves of 5151 dual triode tubes. The difference detecting tubeswould be halves of 6SU7 dual triode tubes and the diodes would be eitherhalves of 6AL5s or semiconductor diodes of any well known type. Thecathode leads of the adding circuit would be 500,000 ohms each, and thecommon plate load 55 would be 17,500. The plate loads of the differencedetecting tubes would each be 250,000 ohms and the common cathode load23 would be 120,000 ohms. The diode load potentiometers 125, 135 and 145would be 1,000,000 ohms. The potential 56 is 450 volts and the potential26, 36 and 46 would be 450 volts with respect to ground.

The same elements are applicable to the circuit of FIG. 2, exceptingthat each of the common loads 155, and 123 would have a value dependingon the number of channels and tubes employed.

Having thus described my invention What is claimed is:

In a circuit for detecting coincident signals in the presence of noisein a communications system having at least three channels; an addingcircuit comprising at least six vacuum tubes, each having grid, cathode,and plate electrodes; a first source of potential with respect toground; a common plate load impedance connecting all of said addingcircuit tube plates to said first source of potential; acathodeiinpedance connecting each of said adding circuit tube cathodesto ground; a difference detecting circuit comprising at least threevacuum tubes, each having grid, cathode, and plate electrodes; a commoncathode impedance connecting all of said difierence detecting tubecathodes to ground; a second source of potential; a plate load impedanceconnecting each of said difference detecting tube plates to said secondsource of potential; at least three rectifier tubes each having acathode electrode connected to the plate electrode of one of saiddifference etecting tubes, and a plate electrode connected through aload impedance to the grid of one of a first group of said addingcircuit tubes; and each of the grids of the difference detecting tubesconnected to one of the grids of a second group of the adding circuittubes.

References Cited in the file of this patent UNITED STATES PATENTS2,293,565 Schock Aug. 18, 1942 2,803,703 Sherwin Aug. 20, 1957 2,934,643Petriw Apr. 26, 1960 2,934,644 Petriw Apr. 26, 1960

